Weak electoral registration requirements are commonly thought to encourage electoral participation, but may also promote electoral fraud. For one, candidates and their supporters can more easily mobilize voters outside the district to register and vote for the candidates, even though these voters do not reside within the district. We statistically detect this classic type of electoral fraud for the first time, by taking advantage of a natural experimental setting in Japanese municipal elections. We argue that whether or not a municipal election was held in April 2003 can be regarded as an “as-if” randomly assigned treatment. The difference-in-difference analysis of municipality-month panel data shows that an increase in the new population just prior to April 2003 is significantly larger in treatment municipalities (with an election) than in control ones (without an election). The estimated effects are, in some cases, decisive enough to change the electoral results, especially when the election is competitive. We argue that our approach – “election timing as treatment” – can be applied to investigate not only this type of electoral fraud but also other “electoral connection[s]” (Mayhew 1974) in other countries.